Mechanical planarization, chemical-mechanical planarization (CMP), polishing, backgrinding, and other abrasive removal processes remove materials from microfeature workpieces at many stages in the production of microelectronic devices. In such abrasive removal processes, an abrasive medium abrades material from the surface of a microfeature workpiece either with or without chemicals. Conventional planarizing or polishing processes are often performed on machines that have a rotary platen, a planarizing pad on the platen, and a carrier assembly for pressing a workpiece against the planarizing pad. To planarize a workpiece, the carrier assembly rotates and translates the workpiece across the surface of the planarizing pad while the platen rotates the planarizing pad. A planarizing solution is generally deposited onto the planarizing pad while the workpiece rubs against the pad surface. The planarizing solution may be a slurry with abrasive particles and chemicals that etch and/or oxidize the surface of the workpiece, or the planarizing solution may be a clean non-abrasive planarizing solution without abrasive particles.
Abrasive removal processes must consistently and accurately produce a uniformly planar surface on the workpiece to enable precise fabrication of circuits and photo-patterns. A non-uniform surface can result, for example, when materials from certain areas of the workpiece are removed more quickly than materials from other areas during processing. In the highly competitive semiconductor industry, it is also desirable to maximize the throughput of such processes by producing a planar surface on a substrate as quickly as possible. This is a function, at least in part, of the ability to accurately stop the process at a desired endpoint. In a typical application, the desired endpoint is reached when the surface of the substrate is planar and/or when enough material has been removed from the substrate to form discrete components (e.g., shallow trench isolation structures, contacts, damascene lines, and other features). Accurately stopping the removal of material at a desired endpoint is important for maintaining a high throughput and good yield because a workpiece may need to be repolished if it is “under-planarized,” or the workpiece may be destroyed or otherwise impaired if it is “over-polished.” Thus, it is highly desirable to stop abrasive processing at the desired endpoint.
One conventional method for endpointing planarization processes is to estimate the polishing rate or polishing period based upon polishing identical substrates under the same conditions. The estimated polishing period for a particular substrate, however, may not be accurate because the polishing rate and other variables may change from one substrate to another and as other parameters change over time. Thus, this method may not produce accurate results. Another method for estimating the endpoint involves removing the substrate from the pad and measuring a change in thickness of a film on the substrate. Removing the substrate from the pad, however, interrupts the planarizing process and may damage the substrate. Thus, this method generally reduces the throughput of CMP processing. Yet another procedure to estimate the endpoint is to measure changes in the friction or drag force between the workpiece and the planarizing pad during the planarizing cycle. The drag force is affected by the type of material at the surface of the workpiece, and thus the drag force changes as different materials are exposed during a planarizing cycle. Such friction-based endpoint procedures are useful, but the measured change in the drag force may not coincide with the actual endpoint on the workpiece because the interface between different films may not be at the endpoint. In such applications the workpiece is over-polished after the endpoint signal based on an empirically determined over-polish period. This method may not precisely and accurately terminate the planarizing cycle at the actual endpoint because differences in the workpieces, condition of the planarizing pad, and other factors that occur throughout a run of workpieces can affect the over-polish period. Therefore, such friction-based endpoint procedures may have only limited utility in many applications.
Still another method for estimating the endpoint involves monitoring changes in reflectance as different materials become exposed at the surface of the workpiece. For example, U.S. Pat. No. 5,433,651 issued to Lustig et al. (“Lustig”) discloses an in-situ chemical-mechanical polishing machine for monitoring the polishing process during a planarizing cycle. The polishing machine has a rotatable polishing table including an embedded window and a planarizing pad with an aperture aligned with the window. The window is positioned at a location over which the workpiece can pass for in-situ viewing of a polishing surface of the workpiece from beneath the polishing table. The planarizing machine also includes a device for measuring a reflectance signal representative of an in-situ reflectance of the polishing surface of the workpiece. Lustig discloses terminating a planarizing cycle at the interface between two layers based on the different reflectances of the materials.
Although the apparatus disclosed in Lustig is an improvement over other abrasive endpointing techniques, it merely provides an indication of when a difference in film type occurs at the surface of the workpiece. The endpoint, however, may not coincide with a change in film type at the surface of the workpiece. As such, this process may work well in some applications, but in other applications the complexity of the processes may prevent such reflectance measurements from accurately endpointing abrasive processes.
One application that is advancing beyond the capabilities of existing endpointing techniques is forming polysilicon contacts in an array. In a typical polysilicon process, the endpoint is detected at a transition from removing only polysilicon to removing polysilicon and nitride. The endpoint signal produced by conventional reflectance-based tools is actually the point at which the nitride begins to be removed, but this typically occurs well before the actual endpoint is achieved. In CMP processes, for example, the optical- or friction-based endpoint techniques can indicate an endpoint at approximately 35 seconds, but in practice an over-polish period of approximately 70 seconds is required to reach the actual endpoint. As a result, conventional endpointing techniques merely provide a pseudo endpoint indication that requires a fixed over-polish time to reach the final endpoint. Even this result, however, may not be accurate because changes in the workpieces or polishing conditions may render the over-polish period inaccurate.
Another challenging application for endpointing is stop-on-nitride planarization in which excess oxide is removed from an array until upper portions of a nitride layer are exposed. Planarization processes typically use a procedure that indicates when the upper portions of the nitride layer are exposed to endpoint this process. At the indicated endpoint, however, the wafers typically have some residual oxide over the nitride array. Conventional processes for stop-on-nitride applications accordingly over-polish the workpieces to reach a final endpoint. The workpieces can further be measured in the periphery to determine a change in the over-polish stage of the process, but this measurement merely assesses the extent of dishing in the periphery instead of the actual removal of material over the arrays on a workpiece. As a result, any changes in the heights of features could result in different thickness measurements in the periphery and thus poor process adjustments. The adjustments made to the over-polish period to achieve the required thickness may thus cause very high dishing because of undetected changes in the stack heights of the features.
Based on the foregoing, existing endpointing techniques may not provide an accurate indication of the true endpoint in many applications. Therefore, it would be desirable to provide methods and apparatus that can identify the actual endpoints or otherwise provide information on the actual status of the surface of the workpiece to improve the efficiency and efficacy of abrasive removal processes.